Besides measuring changes in position of two objects which are movable relative to each other in a lateral direction, there are measuring tasks where it is necessary, exclusively or possibly additionally, to also determine the distance between these objects in a vertical measurement direction perpendicular thereto. Devices for interferential distance measurement, such as are proposed, for example, in the Applicant's patent publication DE 10 2013 203 211 A1, lend themselves for a high-accuracy measurement along such a measurement direction. The devices described in this patent publication provide, at the output, high-resolution phase-shifted incremental signals via which such changes in position can be measured.
A similar device is also known from EP 2 587 212 A2. In the third embodiment and FIGS. 7-9 of this patent application, there is also proposed a way to establish an absolute reference for the incremental measurement. To this end, the beam emitted by a light source is initially directed via collimating optics, a beam splitter, a phase plate, and focusing optics onto a measurement reflector which is movable along the measurement direction. The beam is deflected by the measurement reflector toward a grating in the form of a reflection grating, which finally directs the beam to a detector device formed of two adjacent detector elements which are connected differentially. In the event of a movement of the measurement reflector along the measurement direction, the detector device acquires a signal that is dependent on the position of the measurement reflector and from which a reference signal can be generated at a defined reference position along the measurement direction. The high-resolution incremental signals can then be related to the so-determined reference position, thereby enabling an accurate absolute determination of the position of the measurement reflector along the vertical measurement direction.
However, the disadvantage of the device from EP 2 587 212 A2 recognized by the present invention is that in the event that the measurement reflector is tilted out of its normal position, errors may result in the position determination. This is because the signal phase, which changes during a change in distance along the vertical measurement direction, also changes linearly with the tilt angle.